1. Field of the Invention
The present invention relates to a mobile station and a handover control method.
2. Description of the Related Art
In a mobile communication system using a CDMA scheme, a mobile station generally performs soft handover (SHO) to improve reception quality in a peripheral area of a cell. In SHO, the mobile station communicates with multiple radio base stations (or cells/sectors) with radio links connected therebetween. In general, the radio link between the mobile station and each of the radio base stations is called a branch, and a set of the multiple radio base stations having communications made through the radio links in SHO is called an active set. In addition, branch addition indicates an event where a radio base station, which is to communicate through a radio link, is newly added to the active set, and branch exclusion indicates an event in which a radio base station already communicating through a radio link is excluded from the active set. Moreover, branch replacement indicates an event in which the radio base station having the poorest radio quality in the active set is replaced with a radio base station not included in the active set but having better radio quality when the number of radio base stations in the active set exceeds a predetermined value.
For example, in the WCDMA of 3GPP, a mobile station monitors radio quality (for example, CPICH RSCP, CPICH Ec/N0 and pathloss) between the mobile station and each of the radio base station, makes a determination on branch addition, branch exclusion and branch replacement by using the radio quality, and then reports the determination result to a radio control station. Then, the radio control station makes a control for SHO according to the content reported from the mobile station.
In the 3GPP TS25.331, V6.10.0, the aforementioned operations of the mobile station for reporting the branch addition, branch exclusion and branch replacement are denoted by reporting event 1A, reporting event 1B and reporting event 1C, respectively.
Hereinafter, the operation of the mobile station for the branch replacement will be described with reference to FIG. 1.
In FIG. 1, assume that the maximum number of radio base stations in an active set is 3, and that a foregoing predetermined value for performing the branch replacement is 3. Incidentally, the foregoing predetermined value is defined as a replacement activation threshold in 3GPP TS25.331, v6.10.0.
FIG. 1 shows an example of transitions of common pilot channels' received signal code powers CPICH RSCPs in the mobile station from radio base stations 1 to 4. At a time period of T1 to T2, the mobile station communicates with the radio base stations 1 to 3 by using SHO. In other words, the radio base stations in the active set are the radio base stations 1 to 3. Then, at the time point T2, a value obtained by subtracting the CPICH RSCP of the radio base station 2 having the weakest CPICH RSCP in the active set from the CPICH RSCP of the radio base station 4 exceeds a predetermined threshold Th. Accordingly, the mobile station determines that the radio base station 2 in the active set should be replaced with the radio base station 4, and reports the determination result to the radio control station. In short, the mobile station carries out the reporting event 1C. Here, the threshold Th is defined as H1c/2 (H1c: the hysteresis parameter for the event 1c) in the 3GPP TS25.331, V6.10.0.
Note that a time-to-trigger and the cell individual offsets described in the 3GPP TS25.331, V6.10.0 are set to 0 in the foregoing operation of the branch replacement in order to simplify the description. In addition, the description that “a value obtained by subtracting the CPICH RSCP of the radio base station 2 having the weakest CPICH RSCP in the active set from the CPICH RSCP of the radio base station 4 exceeds a predetermined threshold Th” has the same meaning as that the following formula is true.CPICH—RSCP4≧CPICH—RSCP2+Th 
Here, CPICH_RSCP4 is the CPICH RSCP of the radio base station 4, and CPICH_RSCP2 is the CPICH RSCP of the radio base station 2. Moreover, although only one radio base station is reported for each of two categories of radio base stations, that is, one having a strong CPICH RSCP not in the active set and the other one having a weak CPICH RSCP in the active set, in the foregoing example, multiple radio base stations may be reported for each of the two categories.
Even when a propagation environment is changed, the mobile station can make communications using SHO with appropriate radio base stations by performing the foregoing determinations for the branch addition, the branch exclusion and the branch replacement.
Meanwhile, the standardization of high speed downlink packet access (HSDPA) that is a high speed packet transmission scheme in downlink is in progress in 3GPP, on the basis of the prediction that, along with recent rapid spread of the Internet, the traffic of high speed and large capacity transmission increases especially in downlink because of downloads and the like from databases and websites (see 3GPP TS25.308, v5.7.0, for example).
In downlink of HSDPA, used are a high speed-physical downlink shared channel (HS-PDSCH), a high speed-shared control channel (HS-SCCH) and an associated-dedicated physical channel (A-DPCH). To be more precise, the HS-PDSCH and HS-SCCH are used and shared by multiple mobile stations, while the A-DPCH is allocated to each mobile station individually and is associated with the HS-PDSCH.